Viscosity, and more generally, viscoelasticity, are properties of liquids and solids that relate the shear forces generated by or applied to a material to the amount of shear deformation or flow. While the invention applies equally well to viscoelasticity, the present discussion will be limited to viscosity measurement for simplicity. Viscosity is of widespread interest in many manufacturing environments and is measured as a primary quality of some products and as a secondary quality (a means of monitoring process state) in other processes.
Viscosity describes the force required in order to make successive molecular layers of a liquid move past each other at a given rate of shear (“shear rate”). If one considers a liquid flowing past the walls of a container, the liquid will ideally have no motion relative to the wall at the interface and will have increasingly higher velocities as one observes points successively further from the wall. The shear rate is defined as the gradient of the velocity of the liquid parallel to the surface (meters per second) with increasing distance from the surface (meters). The units of shear rate are 1/seconds. The shear stress is the amount of force per unit area that must be applied in order to cause the motion. While the fluid may have a characteristic flow (and thus a characteristic shear rate) or may be stationary, all measurements of viscosity to date are based on the measurement of shear stress vs. shear rate under an imposed motion of the fluid. Throughout this disclosure, “shear rate at which the viscosity of a fluid” should be taken to mean the shear rate at which the viscosity of the fluid is measured, which may differ substantially from the characteristic shear rate of the fluid in its intended application or point of measurement.
In U.S. Pat. No. 7,007,546 the present inventor detailed a method for measuring the viscosity of a fluid at a selected shear rate, and methods for characterizing characteristics of a fluid by measuring the viscosity of the fluid at selected shear rates. This is achieved by utilizing an Acoustic Wave Device (AWD). The method for measuring viscosity at a desired shear rate utilizes an AWD sensor and an estimate of the shear rate at which the sensor operates, and then modifying the input power level to obtain the desired shear rate. It utilizes measurements of both the input and output power levels and the selection of an input power level by an algorithm. The method of characterizing the fluid characteristics include feeding different levels of power to the sensor and measuring fluid viscosities at the differing power levels. The present invention represents an improvement to the method described in the '546 patent.